In electrophotography, the electrostatic image formed on the photoconductor is developed by the magnetic brush method using either the so called "one-component" developer or "two-component" developer. Usually, the two-component developer system comprises a mixture of relatively fine particles of a toner and relatively coarse particles of a carrier. The toner particles are held on the carrier particles by the electrostatic forces of opposite polarities which are generated by friction of the particles. When the developer comes into contact with an electrostatic latent image formed on the photosensitive plate, the toner particles are attracted by the image and thus make the latter visible. The thus developed image is then transferred onto a recording medium, such as a paper sheet. In the process, therefore, the toner particles should be charged with an accurately controlled amount of static electricity so that they are preferentially attracted to the electrostatically imaged area of the photosensitive plate.
Thus, in turn, the carrier which is used in combination with the toner should fullfil the following criteria: The carrier should have an appropriate triboelectric property which enables it to electrostatically hold the toner particles and to transfer the held toner particles to the electrostatic latent image on the photosensitive plate when contacted. The carrier should have a sufficient mechanical strength to protect the carrier particles from breaking or cracking. The carrier particles should exhibit a good fluidity. The carrier particles should be uniform in their electric and magnetic properties. The carrier should be stable with respect to changes in the environmental conditions, such as humidity. The carrier particles should have a sufficient durability to ensure an acceptable lifetime.
Hitherto, the carrier employed has been selected from a variety of materials. A typical example which has been most widely employed is iron powder. In some cases, the surface of iron is oxidized and/or coated with a resinous material to improve its properties before use. However, iron carriers of this type have not necessarily been satisfactory in respect to the resulting copy quality and the durability.
Ferrite, an oxidic magnetic material, has also been used as a carrier material. Although this ferrite carrier has an acceptable level of durability, its stability with respect to changes in environmental conditions is not sufficient so that the resulting copy quality tends to vary when external conditions such as humidity change. Since ferrite has a high electric resistivity, the copying latitude of the ferrite carrier is narrow and the carrier tends to produce a mono-toned copy. Ferrite may be coated with a resinous material to make it less sensitive to the changes of such environmental conditions as humidity. However, this is not preferred because the coating will increase the resistivity and make the latitude even narrower and also will add to the production costs.